Atoms are made up of tiny particles called protons, neutrons, and electrons. Protons and neutrons are located in the center of the atom in the nucleus, while electrons orbit around the nucleus. Over time, scientists developed models of the atom, starting with plum pudding model where electrons were embedded in a positive sphere, then Rutherford's model with a small dense nucleus at the center, and Bohr's model where electrons orbit in fixed shells around the nucleus. The number of protons determines the element, while the number of neutrons varies between isotopes of that element.

2. “Atoms are said to be the very tiny particles of any
matter that cannot be divided any more after a
certain stage “ for example :
Take a chair and keep on doing its pieces , a stage
will come when there will small particle left that
particle can be said to be an atom.

4. According to today's chemistry I know that
atom has 3 types of particles and they are :
1. Electron
2. Proton
3. Neutron

5. Particle Charge Symbol Location
on the in atom
particle
Electron -1 unit e- Outside
the
nucleus
Proton +1 unit p+ In the
nucleus
Neutron No n0 In the
charge nucleus

6. As we make it from the name that I must be talking
about the mass of the atom taken. The mass is
nothing but the sum of electrons Protons &
neutrons . But actually mass no. can be finded
through just doubling the atomic no. of the given
atom if the atomic no. is even .
If the atomic no. is odd then just double the atomic
no. and add 1 to the answer .

7. Atomic no. is denoted with ‘X’ , mass no with ‘A’
and atomic no. with say ‘Z’.
Atomic no. is written in downward and mass no.
is written in upward form.
Therefore :
Mass no. = A
X= symbol of element
Atomic no.=z

8. Now if we know the atomic no. then we can find
both of the proton & electron.
Because the no. of protons and electrons are
directly proportional to the atomic no. for
example :
We take sodium . The atomic no. of Sodium (Na) is
11 therefore the :-
Atomic no. = no. of Protons = no. of Electrons

9. There is very easy and simple way to find neutrons
as we had earlier taken sodium (Na) so the
formulae for finding neutrons is :-
Mass no. – Atomic no. = no. of neutrons

11.  Eugene Goldstein noted streams of
positively charged particles in cathode rays
in 1886.
 Particles move in opposite direction of
cathode rays.
 Called “Canal Rays” because they passed
through holes (channels or canals)
drilled through the negative electrode.

12. Canal rays must be positive.
Goldstein postulated the
existence of a positive
fundamental particle called
the “proton”.

13. Thomson’s Experiment And
Discovery of Electrons
- Voltage source +
 Passing an electric current makes a beam
appear to move from the negative to the
positive end.

14. Thomson’s Experiment
Voltage source
+
-
 By adding an electric field he found
that the moving pieces were negative.

15. The electron was discovered in
1897 by Thomson. He imagined the
atom as a “raisin pudding” with
electrons stuck in a cake of positive
charge.

16.  Plum Pudding Model,
1896
 Thought an atom was
like plum pudding
 Dough was cloud
 Raisins were electrons
 Didn’t know about
neutrons at this time

17.  English physicist Ernest Rutherford (1911)
 Shot alpha particles at fluorescent screen.
 When an alpha particle hits a fluorescent screen, it
glows.

18. Fluorescent
Lead Uranium Screen
block
Gold Foil

19. What he expected
He Expected
The alpha particles to pass through without
changing direction very much.

20. What he got

21. He thought the mass was
evenly distributed in the atom

22. Since some
particles were
deflected at
large angles,
Thomson’s
model could
not be correct.

23. How Rutherford
explained results…..
 Atom is mostly
empty space.
 Small dense,
positive piece at
center is
(NUCLEUS)

24.  In 1912, Rutherford discovered the nucleus by doing
scattering experiments. He concluded the atom was
mostly empty space, with a large dense body at the
center, and electrons which orbited the nucleus like
planets orbit the Sun.

25. The model created by Rutherford had
still some serious discordance.
According to the classic science, electron
moving around the nucleus should emit
an electromagnetic wave. Electron
should than move not by the circle but
helical and finally collide with the
nucleus. But atom is stable.

26. Rutherford also realized that the
nucleus must contain both neutral and
positively charged particles. The
neutron was then discovered in 1932 by
Chadwick.

27.  Similar to Rutherford’s
model
 Thought atom was mostly
empty space
 Neils Bohr, 1913
 Nucleus in center is
dense, positively
charge
 Electrons revolve
around the
nucleus.

28. Following Rutherford’s planetary
model of the atom, it was
realized that the attraction
between the electrons and the
protons should make the atom
unstable
Bohr proposed a model in which
the electrons would stably
occupy fixed orbits, as long as
these orbits had special
quantized locations

29. Parts of an Atom
Each element has a different number of protons
in its nucleus
Protons have positive charge p
Change the number of protons  change
elements
This is called nuclear physics
The element also has the same number of
electrons
Electrons have negative charge
e
Change the number of electrons  ionize the
element
This is called chemistry
Some elements also have neutrons
Neutrons have no charge n
They are in the nuclei of atoms